This invention relates to alternating current machine drive systems employing current feedback pulse width modulation control and more specifically relates to a novel control link for such systems which suppresses resonance current in the line and between the line inductance and the d-c filter capacitance.
Alternating current machine applications which require regulation of machine speed and torque are well known. Such applications frequently employ inverter machine drive systems for their control. A typical control system of this kind is disclosed in U.S. Pat. No. 4,320,331, issued Mar. 16, 1982, entitled "TRANSISTORIZED CURRENT CONTROLLED PULSE WIDTH MODULATED INVERTER MACHINE DRIVE SYSTEM" in the name of Allan B. Plunkett and assigned to General Electric Company. The control system of the above patent employs a circuit consisting of pairs of transistors coupled in series-aiding fashion. Each of the pairs of serially coupled transistors is connected across a d-c filter capacitor which is in turn connected across the d-c terminals of a suitable rectifier. The rectifier input is connected to a suitable a-c power source. Electromagnetic interference filters may also be provided in the a-c input circuit. The transistors of each inverter transistor pair are alternately conductive to supply alternating current at the inverter output in accordance with a current error signal which is proportional to the difference in magnitude between the actual inverter phase current and a sinusoidal reference signal. The conduction state of the transistors of each pair reverses each time the current error signal exceeds an upper and lower hysteresis limit, which limits vary in accordance with actual inverter phase current. The inverter output voltage which controls machine speed and torque is then controlled by varying the sinusoidal reference signal frequency and amplitude, respectively, in accordance with operator commands. Optimum inverter machine drive performance is achieved by regulating this sinusoidal reference signal amplitude and frequency in accordance with a feedback machine torque signal and a feedback machine flux signal, respectively.
This type system effectively isolates the machine current from variations in the d-c filter voltage. However, it is still possible for the line inductance, including the inductance of the electromagnetic interference filter, and the d-c capacitor to resonate. Thus, the input a-c power line current may experience very wide oscillation which makes the control system unusable in many applications. The line current resonance also causes additional stress on the system components due to the voltages which are produced by the resonating current.
In the past, this line resonance was reduced by increasing the size of the d-c filter so that line current would be better smoothed. This, however, requires a substantially larger d-c filter capacitor than would otherwise be needed and also requires the use of a relatively large d-c smoothing reactor.
It is an obejct of the present invention to provide a control circuit for suppressing line resonance current in a current feedback pulse width modulation system while allowing minumum d-c filter operation.
It is a further object of the present invention to provide a control circuit for suppressing line resonance currents to improve the line power flow by maintaining the instantaneous line current at the minium required to support the instantaneous power flow.